Sailcloth is typically made from a variety of materials, with one of the most common being a tightly woven cloth of polyester yarns. Sailcloth is one of the most tightly woven textiles in the world and requires extensively modified heavy looms to generate the necessary forces to attain such a dense construction. Normally, polyester sailcloth is only woven in what is known as a plain weave, in which every warp yarn passes over and under each fill yarn, with the yarns being crimped over each other. After weaving, the cloth is impregnated with a resin and heated, causing the resin to cure and also causing the polyester fabric to shrink.
The above described weaving method tends to impart certain characteristics to the cloth due to the nature of the operation itself. The warp yarns, which run in the machine on long direction tend to crimp more than the weft or fill yarns, which run in the cross machine direction. Sails formed from such cloth are made up of a number of joined panels, and it is desirable to align the yarns with less crimp along directions of maximum stress or load in the sail. This, in turn, reduces stretch, which would otherwise cause the sail to lose its ideal or designed shape when subjected to increasing wind forces.
Fill oriented cloth imposes limitations on how panels can be cut and arranged in a sail while still making efficient use of the cloth. A common design using fill oriented cloth is a so-called cross cut design, in which the seams are substantially horizontal, and the fill yarns run from the top to the bottom of the sail.
Studies of the properties of sails have demonstrated that in triangular sails, especially genoas or jibs, the main forces radiate out of the corners of the sail. See e.g., FIG. 4.